star birth ast 112 lecture 9. star birth the milky way has 200-400 billion stars. 2-3 stars born per...
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Star Birth
AST 112 Lecture 9
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Star Birth
• The Milky Way has 200-400 billion stars.
• 2-3 stars born per year in our galaxy!
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What is this?
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Star Birth• Star birth occurs in
clouds of gas and dust that tend to be:
– Cold • Molecules are slow, don’t
resist collapse
– Dense• Lots of stuff to make stars
out of
• Called molecular clouds
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Cold, Dark Clouds
• How do we know the dark spot isn’t just devoid of stars?
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Cold, Dark Clouds
• Look at the edges. Stars are red and gradually fade away.
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Cold, Dark Clouds
• Dust doesn’t scatter infrared as effectively as visible light
• See stars behind cloud if we look in infrared
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Star Birth
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Star Forming Clouds
• -450 (that’s negative!) oF– “Absolute Zero Temperature” is at -459 oF
• 300 molecules per cm3
– Earth’s atmosphere at sea level has 1018 more molecules per cubic centimeter
• “Lumpy”
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Star Clusters
• Most stars form in clusters– Clouds have masses of hundreds of Msun
– Form open clusters of 100+ stars
• Rarely, but sometimes:– Very small, dense clouds collapse to form one or a
few stars
• We’ll assume a cluster is forming
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Cloud Collapse
• Gravity pulls material inward
• As the cloud collapses, it gets warmer
• Causes pressure increase, resists collapse. BUT:
• Warm molecules release photons, cooling the cloud– This happens so long as the cloud isn’t too dense– Pressure can’t resist collapse yet due to cooling
• Collapse continues
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Cloud Fragmentation
• The cloud fragments
– At the Jeans Mass, gravitational collapse happens very quickly
– Dense areas of the cloud reach the Jeans mass first
• These fragments eventually form protostars
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A Fragment Forms a Protostar
• Eventually, can’t radiate heat because too dense– Too many molecules to run into
• Eventually, almost all of the radiation is trapped
• Heat goes up, pressure goes up – contraction slows
• It is now a protostar.
– Bright but no nuclear fusion
– They are still embedded in the molecular cloud
– How can we hope to observe them?
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Protostars emit heavily in infrared, which travels through dust.
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Protostar to Main Sequence
• It moves to the Main Sequence of the HR diagram– Spends most of its life here – Millions to billions of years
• The O / B stars can actually die before the M stars turn on!
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Why so blue?
• Open clusters often look very blue. Why?
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…red stars?
• This cluster is 50 million years old
• Red stars turn on in about 150 million years
• Why are they there?
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Back to the Cloud
Credit: ESO
As the cloud collapses and stars turn on, it begins to glow.
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The Eagle Nebula
• A site of active star formation
• The entire cloud glows from star formation– Hydrogen glowing red
• Young, bright blue stars are visible Credit: ESO
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Pillars of Creation
• Stars outside of these columns are “boiling” gas off the top
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Pillars of Creation
• EGGs (Evaporating Gaseous Globules)
• These are protostars that get uncovered as surroundings boil away
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Trifid Nebula Orion Nebula
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Trifid Nebula (up close)
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Cloud to Cluster
• Stars slowly clear out surroundings
• Gas cools
• Starts to look less like chaotic cloud, more like organized cluster
Credit: NASA
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Young Star Cluster
• Young stars still surrounded by dust and gas
• We see a reflection nebula– Blue light scattered more
stronglyNGC 346 (In Small Magellanic
Cloud)
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Open Cluster
• Later on, just looks like a group of stars
• Eventually disperse, “mix in” with the galaxy
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Newborn Star Size
• Largest size?
• Usually top out at 100 MSun
• Pistol Star (150 MSun)
• Excessive radiation pressure drives outer mass away
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Newborn Star Size
• Smallest size?
• 0.08 solar masses
• Anything less can’t squish hard enough
YOU FAIL. GO HOME.
Less than 0.08 solar masses: Brown Dwarf
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Brown Dwarfs
• Brown dwarfs are actually either deep red or infrared
• Try to catch them when they form because they cool off– Best to look in star
forming regions
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Newborn Star Size
• Typical size?